Micro-electromechanical systems (MEMS) are systems which are developed using thin film technology and which include both electrical and micro-mechanical components. MEMS devices are used in a variety of applications such as optical display systems, pressure sensors, flow sensors and charge control actuators. MEMS devices use electrostatic force or energy to move or monitor the movement of micro-mechanical electrodes which can store charge. The size of a gap between the electrodes is controlled by balancing an electrostatic force and a mechanical restoring force. Digital MEMS devices use two gap distances, while analog MEMS devices use multiple gap distances.
MEMS devices have been developed using a variety of approaches. In one approach, a deformable deflective membrane is positioned over an electrode and is electrostatically attracted to the electrode. Other approaches use flaps or beams of silicon or aluminum which form a top conducting layer. With optical applications, the conducting layer is reflective and is deformed using electrostatic force to scatter light which is incident upon the conducting layer.
These approaches suffer from electrostatic instability which results in a greatly reduced range of motion. The instability occurs when a voltage controlling the electrodes is increased to control the gap distance. Since the electrodes form a variable capacitor, charge runaway results when the capacitance is increased due to decreasing gap distance. As the capacitance is increased, more and more electrical charge is pulled onto the capacitor, resulting in charge runaway. Since the amount of charge stored on the capacitor is not controlled, control of the electrode movement is possible for only about ⅓ of the total gap distance, because outside of this range the electrode will “snap down” to mechanical stops. Thus, a non-linear relationship exists between the electrode voltage and electrode displacement over a large range of gap distances. This inability to control the gap distance for more than about ⅓ of the total gap distance limits the utility of the MEMS devices. For example, with optical display systems, interference or defraction based light modulator MEMS devices preferably should have a large range of gap distance control in order to control a greater optical range of visible light scattered by the optical MEMS device.